1. Field of the Invention
The present invention relates to an optical scanning apparatus for scanning a surface to be scanned by a spot of a beam of light focused on that surface by deflecting that spot, and more particularly, to an optical scanning apparatus of the type in which the scanning position of the spot of the light beam is detected to generate a synchronizing signal for scanning.
2. Description of the Related Art
Optical scanning apparatus for scanning a surface to be scanned by the spot of a beam of light focused on that surface by deflecting the beam of light are for use in a recording or reading apparatus.
The optical scanning apparatus for use in a light beam printer, such as a laser beam printer, incorporates a light beam detection unit for detecting the scanning position of the spot of the light so as to allow the generation of the beam of light to be modified in response to image information to be recorded. In this light beam detection unit, a light-receiving surface of a photoelectric conversion device is generally disposed inside a deflection area of the light beam and outside a recording area, and a beam of light irradiating this light-receiving surface is photoelectrically converted to generate a synchronizing signal.
FIG. 10 shows a conventional optical scanning apparatus. A beam of light emitted from a light source (not shown) is deflected by a polygonal mirror 322 which is rotated at a fixed speed to create a beam of light 301b, and this deflected beam of light 301b is focused on a surface 100a to be scanned by an image forming optical system which consists of image forming lenses 331a and 331b having f.theta. characteristics so as to form a spot of light. The surface to be scanned 100a is scanned by this light spot. A beam of light 301c which is deflected to the outside of the recording area is reflected by a plane mirror 398a when it reaches a predetermined synchronizing position and then received by a light-receiving surface of a photodiode 348 which is used as the photoelectric conversion device. The photodiode 348 converts this beam of light into an electrical signal and generates a synchronizing signal.
For the purpose of improving the accuracy of detection, the light-receiving surface of the photodiode generally has a small area, and the beam of light is focused for a small, sharp spot. Thus, inaccurate alignment of the light spot on the light-receiving surface causes a reduction in the detection accuracy or disability of detection. A first cause of these problems may be a facet error of the polygonal mirror 322. If any of specular surfaces of the polygonal mirror 322 is inclined with respect to the rotation axis thereof due to the manufacturing errors, the beam of light deflected (reflected) by the associated specular surface varies in a direction perpendicular to the direction of scanning, causing a shift of the scanning path of the beam of light with respect to the center of the light-receiving surface and thereby making it impossible for the beam of light to be received on the light-receiving surface precisely. The second cause of the problems is a shift of an optical axis due to the errors of installation of the light source, the image forming lenses 331a and 331b, the reflecting plane mirror 398a and the photodiode 348. Fine adjustment of these components is very difficult in a case where the beam of light is focused as a small, sharp spot and where the light-receiving surface of the photodiode 348 is small for high-precision. A shift of the optical axis also occurs due to the strains of a supporting member which is caused by shortage of the rigidity of the supporting member and the beam of light cannot therefore be received on the light-receiving surface precisely.
In order to solve the problems caused by the second cause, Japanese Patent Laid-Open No. 59-188615 proposes a method of expanding the beam of light 301c which is to be received on the light-receiving surface of the photodiode 348 by a cylindrical convex mirror in a direction perpendicular to the direction of scanning to form an elongated spot of light.